1. Field of the Invention
The present invention relates to an optical print head using a light emitting element array and a rod lens array and an image forming apparatus using the optical print head as an exposing unit.
2. Description of the Background Art
Today, there is an increasing demand for a miniature, digital optical writing unit that meets the current trend toward a miniature digital copier, printer, digital facsimile apparatus or similar digital image forming apparatus. Digital writing is practicable with either one of an optical scanning system and a solid-state writing system. The optical scanning system uses a semiconductor laser or similar light source, an optical deflector, and a lens. The optical deflector deflects light issuing from the light source while the lens focuses the deflected light in the form of a beam spot. The solid-state writing system uses an LED (Light Emitting Diode) array, organic EL (Electro Luminescence) device array or similar light emitting element array. A focusing element array focuses light issuing from the light emitting element array in the form of beam spots. While the optical scanning system needs a long optical path due to the deflector, the solid-state writing system makes the optical path extremely short and is therefore suitable for the miniaturization of an optical writing unit.
Generally, the focusing element array included in an optical print head is implemented as a rod lens array having a number of graded-index rod lenses arranged in two or more arrays. Japanese Patent Laid-Open Publication No. 5-138934, for example, proposes a method of producing an optical print head. This method equalizes a distance between a light emitting element array and an image carrier and the conjugate length of the individual rod lens of a rod lens array. In this condition, the rod lens array can be accurately positioned at the center between the light emitting element array and the image carrier.
However, optical simulations for measuring a beam spot diameter with respect to a defocus showed the following. When the distance between the light emitting element array and the image carrier was the same as the conjugate length of the individual rod lens, the beam spot diameter could not be sufficiently reduces. Moreover, a beam waist where the beam spot diameter becomes smallest with respect to the defocus was shifted from the image carrier.
On the other hand, Japanese Patent Laid-Open Publication No. 6-320790 discloses an optical print head constructed to guarantee MTF (Modulation Transfer Function) of 50% or above for irregularity in the height of an emitting region (+0.2 mm). For this purpose, the above document teaches that a distance between the average height of the emitting regions of LEDs and the center of a lens array is greater than the resonance length of the lens array. This kind of scheme has the following problem left unsolved. Assume that the distance between the average height of the light emitting regions of LEDs and the center of the lens array is greater than one half of the conjugate length of the rod lens array. Then, MTF is lowered although it varies little with respect to irregularity in the height of the emitting region, failing to increase the resolution of the focusing element array.
It is therefore an object of the present invention to provide an optical print head capable of increasing resolution by reducing a beam spot diameter and positioning a beam waist in the vicinity of an image carrier.
It is another object of the present invention to provide an image forming apparatus using the above optical print head as an exposing unit.
In accordance with the present invention, an optical print head includes a light emitting element array having a plurality of light emitting elements. A rod lens array has a plurality of rod lenses for focusing light output from the rod lenses on an image carrier in a form of beam spots. The rod lenses each have a conjugate length greater than a distance between the light emitting element array, which forms a x1 image, and the image carrier.
Also, in accordance with the present invention, an image forming apparatus includes an image carrier for forming a latent image thereon, and an exposing unit including an optical print head for forming the latent image on the image carrier. The optical print head has the above-described unique configuration.